Signal Regeneration from Telegraph to T1
From the day’s of Morse’s telegraph to modern T1 lines, regenerative repeaters have made long distance wireline communication possible
By: John Shepler
You probably realize that there is a distance limitation to sending signals through wires, fiber strands or wirelessly. Your USB and Ethernet cables can be just so long. Yet, we easily communicate worldwide on the Internet and through private networks. How’s that possible?
Signals Get Weaker With Distance
Signals do deteriorate as they travel. The trick is to boost them from time to time and recreate the original waveform. Analog phone lines use passive equalizer components, capacitors and coils, to compensate for the reactance in the wires that causes the voice signals to sound muffled before they get to the central office. Over longer distances, linear amplifiers boost signals that lose amplitude due to the resistance in the wiring.
Boost the Signal, Boost the Noise
There’s one small problem with simply amplifying a signal to bring it back to its original voltage. Amplifiers amplify everything. That includes the signal we want, but it also includes any noise that is picked up along the way. There’s lots of noise that leaks into the wiring. Twisting the pairs of copper wire helps cancel out external fields, but you still get small amounts of power line hum, crackles from thunderstorms, and crosstalk from other wires in the bundle. You boost the faded signal, but you boost the noise by the same amount reducing the ratio of signal to noise on the line. The longer the path, the noisier your signal gets at the end.
Digital Can Eliminate Noise
Digital transmission offers the opportunity to get around the deteriorating signal to noise problem. Digital signals have only two states, 1 and 0. These can be low and high currents, different polarity voltages or the presence or absence of a light pulse. It doesn’t matter as long as there is a way to tell the difference between the ones and the zeros. If so, you can recreate the original signal and throw out the noise completely.
Here’s a simple example. In the days of the telegraph, the Morse code dots and dashes could only be sent so far using the wires and batteries of the day. One solution is to have telegraph stations with live operators along the line between cities. The first station would send the message to the second, where it would be transcribed and then resent to the next station in line. Because the signal was regenerated completely, it would be exactly the same leaving the second station as it was leaving the first.
Electrical Relays To The Rescue
Well, in theory anyway. Any mistakes would show up as noise in the signal that couldn’t be removed. Besides being slow and potentially error-prone, this method is laborious. An automatic process is to have a telegraph receiver (sounder) connected to a key and re-transmit everything it receives. This is the familiar electrical relay. It is also a regenerative repeater. The relay repeats everything it receives and regenerates the signal back to its original form.
T1 Is Today's Digital Line
Sound old-timey? We still use this process for T1 lines. No, there are no Morse code keys in those telephone boxes. It’s all solid state electronics these days. The similarity between Morse code and T1 digital transmission is that they both use binary signals. The T1 line is running at about 1.5 Mbps, a bit fast for telegraphers but no problem for a solid state regenerator. The shape of the signal at the input has been distorted by electrical characteristics of the line and attenuation over distance. It has also picked up noise.
The regenerative repeater determines whether each pulse is a one or a zero and then recreates the string of pulses in the T1 signal at exactly the specified voltage level and waveform of a “perfect” T1 signal. You can’t tell the difference of a T1 signal leaving the telco office and one leaving a regenerative repeater. These repeaters can be placed every 6,000 ft to regenerate the signal back to spec. Five or ten miles out, it looks as good as it did at the central office.
Why T1 Is Available Everywhere
The availability of simple, inexpensive regenerative repeater cards is what makes T1 service so universally available. It doesn’t matter if your business is located downtown or on a farm in the middle of nowhere. If you can get telephone service, you can probably get T1 service. T1 takes two pair of phone wires, one for upload and one for download. Other than that, regular multi-pair telephone cable is all that is required. If no T1 service has ever been installed for miles around, the provider may have to do some construction to install those regenerative repeaters to get the signal to your location.
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